THE  HYDROLYSIS  OF  TERTIARY 
ARSENATES 


BY 

HAROLD  BYRON  JOHNS 


THESIS 

FOR  THE 


DEGREE  OF  BACHELOR  OF  SCIENCE 

TN 

CHEMICAL  ENGINEERING 


COLLEGE  OF  LIBERAL  ARTS  AND  SCIENCES 


UNIVERSITY  OF  ILLINOIS 


1922 


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Acknowledgement 

The  writer  wishes  to  express  his  deep  feel- 
ing of  gratitude  to  Dr  0 J.H, Reedy,  whose  careful  su- 
pervision made  this  work  possible,  and  to  thank  him 
for  the  assistance,  which  he  so  generously  gave  at 
all  times. 


Digitized  by  the  Internet  Archive 
in  2015 


https://archive.org/details/hydrolysisoftertOOjohn 


Table  of  Contents  . 

Page 

I . Introduction  1 

(a).  Pacts  suggesting  the  problem  1 

(b  ) . Pteview  of  the  literature  2 

(c).  Statement  of  the  problem  3 

II.  Theoretical  3 

(a) .  Effect  of  colloidal  dispersion  on 

the  toxicity  of  an  insecticide  3 

(b) .  Permanence  of  suspension  3 

(c) .  Protective  effects  against  weathering  3 

III  . Experimental  4 

(a) .  Preparation  of  the  colloid  4 

(b ) • Preparation  of  other  arsenates  5 

(c) .  Determination  of  the  solubilities  of 

the  arsenates  6 

IV.  Summary  8 

V,  Bibliography  , 10 


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1. 


I. 

Introduction. 

(a).  Facts  suggesting  the  problem. 

Lead  arsenate  has  been  used  as  an  insecticide  for  several 
years.  On  account  of  the  increased  cost  of  manufacture,  how- 
ever, manufacturers  have  recently  directed  their  attentions  to 
the  manufacture  of  a substitute,  calcium  arsenate,  which  can  be 
produced  for  about  one  half  the  cost  of  the  lead  arsenate. 

Calcium  arsenate  is  now  being  manufactured  in  large  quan- 
tities and  is  used  in  the  south,  principally  for  the  destruc- 
tion of  the  cotton  boll  worm  and,  to  a lesser  extent,  for  the 
tobacco  worm,  Colorado  potato  beetle,  and  the  codling  moth. 

It  has  been  conclusively  demonstrated  that  these  two  arsenates 
are  equal  in  toxic  value  and  killing  power. 

There  is  one  great  disadvantage  in  the  use  calcium  arsen- 
ate as  an  insecticide  in  that,  even  though  the  product  may  be 
free  from  water  soluble  arsenic  when  it  leaves  the  factory,  it 
very  often  has  an  appreciable  of  water  soluble  arsenic  when  it 
reaches  the  consumer.  This  water  soluble  arsenic  burns  the 
foliage  of  plants  and  since  it  cannot  be  removed,  large  amounts 
of  the  insecticide  are  returned  and  cause  a great  loss  to  the 
manufacturer.  Another  disadvantage  is  that  many  batches  are 
spoiled  in  preparation  because  of  the  difficulty  of  properly 
controlling  the  conditions  of  formation. 


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2. 

(b ) , Review  of  the  literature. 

Reedy  and  Haag  (l)  found  that  the  presence  of  such  impu- 
rities as  HaCl  or  FeSO^  seemed  to  catalyze  the  formation  of  wa- 
ter soluble  AS2O5,  Exposure  to  air  and  carbon  dioxide  had  sim- 
ilar effects  . 

De  Toni  (2)  recently  prepared  colloidal  tri -calcium  phos- 
phate by  adding  slowly  and  with  constant  stirring,  nearly  boil- 
ing N Na^PO^  solution,  largely  diluted,  to  an  equal  volume  of 
hot  N GaGlg,  which  also  had  been  largely  diluted  and  had  been 
mixed  with  the  desired  quantity  of  gelatin  as  protective  col- 
loid. He  found  that  cold  solutions  react  slowly  and  remain 
clear  for  a long  time.  By  adding  the  phosphate  to  the  calcium 
chloride  an  excess  of  Ga  ions  is  assured  and  the  formation  of 
tri -calcium  phosphate*  Gelatin  gives  a precipitate  with  the 
sodium  phosphate  and  is  therefore  added  to  the  calcium  chloride. 
De  Toni  prepared  the  Na^PO^  solution  by  adding  the  calculated 
amount  of  NaOH  to  a solution  of  Ha2HP04,  diluting  to  normal  and 
then  carefully  protecting  from  the  air.  By  varying  the  concen- 
trations he  found  that  to  make  colloidal  solutions  containing 
2.068,  3.102,  4.137  g.  Ga3(P04)2  in  1000  cc.  none  of  which  would 
show  a precipitate,  there  were  required,  respectively,  9.5,  17.0, 
and  27.5  g.  of  gelatin.  He  found  that  gum  arabic,  blood  serum, 
and  starch  may  be  used  as  protective  colloids,  but  sugar  and 
caramel  do  not  give  colloidal  tri -calcium  phosphate. 


3. 

(c).  Statement  of  the*  problem. 

Since  there  is  a possibility  that  a precipitate  in  col- 
loidal form  would  not  be  so  apt  to  hydrolyse,  the  purpose  of 
this  research  was  first,  to  try  to  prepare  colloidal  tri-calcium 
arsenate;  second,  if  this  colloid  is  obtained,  to  study  its  sta- 
bility and  ionization;  and  third,  to  study  the  stability  and  ion- 
ization of  other  tertiary  arsenates. 

II . 

Theoretical . 

(a).  Effect  of  colloidal  dispersion  on  the  tox- 
icity of  an  insecticide. 

Burton  (3)  found  that  the  average  diameter  of  the  colloid- 
al particles  of  gold,  silver,  and  platinum  range  from  0.2  to  0.6 
micron  and  other  colloids  are  known  to  have  particles  that  are 
only  one  millimicron  in  diameter.  These  particles  are  therefore 
much  smaller  than  the  particles  of  an  ordinary  precipitate  which 
is  in  suspension.  Hence,  since  we  have  increased  solution  and 
smaller  particles,  the  toxicity  of  an  insecticide  should  be  in- 
creased when  it  is  in  a colloidal  form. 

(b  ) . Permanence  of  suspension. 

ciiy'iyKpyy'iJ  f 

Tfhen  peptized,  colloids  remain  suspended  or  disbursed  for 
an  indefinite  period  of  time.  On  the  other  hand  certain  sus- 
pensions are  often  settled  within  a few  hours. 

(c).  Protective  effects  against  weathering. 

If  a colloidal  solution  of  tri-calcium  arsenate  could  be 
used,  it  would  not  only  have  an  increased  toxicity,  but  it  would 
also  tend  to  gelatinize  on  the  foliage  of  the  plant  on  which  it 


4. 

is  sprayed.  This  gelatinous  form  would  not  be  so  easily  washed 
off  by  the  rain  and  would  therefore  materially  reduce  the  num- 
ber of  sprayings  necessary  each  year.  It  might  be  thought  that 
this  gelatinous  coating  would  cover  the  pores  of  the  leaves  and 
in  this  way  interfere  with  the  respiration  and  transpiration  of 
the  leaves.  However,  sprays  are  not  often  applied  so  heavily 
but  what  the  liquid  collects  in  tiny  droplets  instead  of  form- 
ing a solid  coating. 

Ill  . 

Experimental, 

(a).  Preparation  of  the  colloid. 

A calculated  amount  of  KOH  solution  was  added  to  a solu- 
tion of  KH2AsO^  to  form  K^AsO^  solution.  This  solution  was  fil- 
tered and  diluted  to  3N,  A solution  of  GaOlg  was  made  up,  fil- 
tered, and  diluted  to  N. 

Five  cc.  of  the  3N  potassium  arsenate  solution  were  dil- 
uted to  500  cc.  and  15  cc.  of  the  N.  calcium  chloride  solution 
were  diluted  to  500  cc.  Seventeen  g.  of  gelatin  were  added  to 
the  calcium  chloride  solution  and  with  both  solutions  near  boil- 
ing, the  arsenate  solution  was  added  slowly  to  the  calcium  chlor- 
ide and  gelatin  solution.  The  stirring  was  continued  for  a few 
moments  after  the  KjAsO^  solution  had  been  added.  This  gave  a 
solution  or  suspension  which  was  white  by  reflected  light  and 
yellowish  color  by  transmitted  light.  This  solution  gelatinized 
after  standing  about  20  hours.  When  diluted  with  two  parts  of 
water,  it  gave  a clear,  water  white  solution,  which  did  not  ’gel- 
atinize and  which  remained  clear  for  several  days  . An  increased 


5. 


amount  of  gelatin  made  a more  viscous  jelly.  When  the  solutions 
were  heated  to  about  the  melting  point  of  gelatin  (40®  G.),  in- 
stead of  100®  G.  the  product  appeared  to  be  the  same  as  was  ob- 
tained in  the  other  case.  Ga(N03)2  was  tried  in  place  of  the 
GaGl2  but  the  jelly  did  not  seem  to  remain  viscous  quite  as  well. 

An  attempt  was  also  made  to  make  the  colloidal  Ga3(As04)2 
by  dropping  H^asO^  into  a paste  of  Ga(0H)2  similar  to  the  method 
recommended  by  Reedy  and  Haag  (l).  The  gelatin  was  dissolved  in 
the  arsenic  acid.  This  method  proved  unsuccessful  because  the 
gelatin  seemed  to  slow  the  reaction  too  much.  Even  when  the  so- 
lution was  made  distinctly  acid  (phenolphthalein ) , by  the  time 
the  upper  part  of  the  solution  had  gelatinized,  the  lower  part 
would  be  decidedly  colored,  showing  that  the  OH  ions  were  again 
in  excess  . It  was  thought  at  first  that  this  formation  of  OH 
ions  indicated  that  hydrolysis  was  taking  place  but  the  fact 
that  only  a fraction  of  the  calculated  amount  of  arsenic  acid 
was  necessary  to  make  the  solution  acid  seems  to  show  rather 
conclusively  that  it  was  the  slowness  of  the  reaction  which 
caused  the  phenomena. 

The  above  concentrations  give  a 0.015  H solution  of  Ga^- 
(As0^)2  or  0.995  g.  of  tri-calcium  arsenate  per  liter. 

(b).  Preparation  of  other  arsenates. 

Galculated  amounts  of  the  K^AsO^  solution  were  added  to 
solutions  of  AgNOs,  Pb  (1103)2,  and  Gu(N03)2  to  form  Ag3As04, 
Pb3(As04)2,  and  Gu3(As04)2  respectively.  In  each  case  the  prod- 
uct was  filtered,  thoroughly  washed,  and  then  dried  on  a clay 
plate.  Each  product  was  kept  in  a stoppered  sample  bottle. 


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6 • 

(c).  Determination  of  the  solubilities 
of  the  arsenates  . 

The  solubilities  of  the  arsenates  were  determined  by  meas- 
uring their  electromotive  forces,  when  used  as ’half  cells  with 
silver  electrodes,  and  then  calculating  the  ion  concentrations 
from 'the  equation: 

E = 0.0585  log  G’/C" 

The  half  cells  (used  against  N/10  Gal.)  were: 

Ag/AgjAsO^/SN  Kl'iOj 
Ag/aggAsO^/Pb^  (AsO^  )2/3N  KiJOj 
Ag/Ag^AsO^/GuglAsO^)^^^  KHO^ 

Ag/Agj^AsO^/Gol.  Ga5(AsO^)g/3N  KNO^ 

The  setup  used  was  the  ordinary  one  consisting  of  a stor- 
age battery,  potentiometer  box,  variable  resistance,  galvanom- 
eter, and  standard  cell. 

Gonsidering  E’  as  the  potentiometer  reading  and  using  a 
N/io  AgGl  cell  as  a reference,  the  above  equation  becomes: 

E'  - .0474  r .0585  log  G (AgCl )/C ( Ag3As04 ) 

z .0585  log(A^)AgGl  - ^0585  log  (A^)Ag'^As04 
The  concentration  of  Ag  ions  in  n/10  AgGl  is  1.94  x lO”*^ 
and  after  substituting  this  value  in  the  equation,  it  becomes: 

E’  - .0474  « .0585  log  1.94  x 10”^  - .0585  log  (Aj)Ag3As04 
E’  : .0474  + .0585  log  1.94  x 10"^'  - .0585  log  (Ag)Ag^AsO^ 
Whitby  (4)  found  that  the  solubility  of  Ag^As04  was 
8.5  X 10“^  g.  per  100  g.  of  solution.  This  is  equal  to  8.5  x 
10"^  g.  per  liter  of  solution. 

Ghanging  this  to  moles  it  becom.es: 


7 


8.5  X 10"^/462,6  = 1.839  x 10“5  = (ASO4) 

1.839  X 10-3  X 3 = 5.517  x 10“5  = {A$) 

Hence : 

(Ag)^  X (Aio”)  r (5.517  X 10“^)^  x (1.839  x 10-5) 

: 308  X 10-20 

Therefore: 

(Ag)  r 308  X 10“^ V (AsO^) 

Substituting,  the  equation  becomes: 

E’  = .0474  + .0585  log  1.94  x 10-S  --.0585  x 
log  308  X 10-20/  (ASO4) 

E’  = ,0474  + .0585  log  1.94  x 10“-  - .0585  x log 

308  X 10"^^  + .0585  log  (ASO4) 

Combining  the  terms  which  are  constant  for  this  work,  the 

equation  becomes  : 

E'  = constant  -**  .0585  log  (As5”) 

= 0.67905  + 0.0585  log  (ASO4) 

Log  (AsOJ)  = (E»  - 0.67905.) 

0.0585 

The  following  readings  were  made: 

No.  1.  Silver  arsenate  (to  try  and  check  Whitby's  results). 
No.  2.  Copper  arsenate. 

No.  3.  Lead  arsenate. 

No.  4.  Apiece  of  gelatin  from  a Colloidal  calcium  arsenate 
solution  several  days  old.  (Made  from  Ca(N03)g). 

No.  5.  Some  colloidal  calcium  arsenate  which  had  been  made 
from  calcium  nitrate'}  but  had  been  diluted  with  tv^o  parts  of  wa- 
ter and  therefore  had  never  gelatinized. 

The  above  readings  were  taken  after  the  cell  had  been  made 


1 


1 


8. 

up  for  at  least  24  hours . 

No.  6.  Colloidal  oalcium  arsenate  made  iiith  CaCl2.  This 

reading  was  taken  immediately  after  the  gelatinous  colloid  had 

been  put  in  the  cell. 

No.  7.  No.  6 after  standing  for  72  hours. 

No.  8.  Same  as  No.  6 but  made  with  Ga(NO„Oo  instead  of 

o ^ 

G aC Ig  • 


No.  9.  No.  8 after  standing  for  72  hours. 


Results • 

Reading 

No. 

Re ading 
(Volts ; 

(AsO” 

) 

1. 

0.1820 

3.185 

X 

10“^ 

2. 

0.1828 

3.295 

X 

10“^ 

3. 

0.1933 

5.000 

X 

10"^ 

4. 

0.1155 

2.330 

X 

10-10 

5. 

0 . 12 15 

2,955 

X 

10-10 

6. 

0.0716 

4.100 

X 

10"^^ 

7. 

0.0325 

8.890 

X 

10”^^ 

8. 

0.1566 

1.172 

X 

10“^ 

9. 

0.1425 

6.720 

X 

10’^^ 

IV. 

Summary  . 

Calculating  from  the  solubility  given  by  Whitby  (4),  read- 
ing NO.  1 should  give  the  concentration  of  the  AsO^  ions  as 
1.84  X 10”^  instead  of  3.185  x 10“^. 

This  difference  makes  the  accuracy  of  the  other  figures 
rather  doubtful.  Even  though  the  figures  may  not  be  quite  ac- 
curate, however,  the7/  seem  to  show  that  the  copper  and  lead  ar- 


f 


i 


tt 

I : 


.■  ' 'r-'  C f 

■ i 


9 


senates  are  a little  more  soluble  than  the  silver  arsenate; 
that  colloidal  calcium  arsenate,  especially  when  prepared  with 
GaCl2,  is  even  more  insoluble  than  the  silver  arsenate;  and  that 
none  of  these  arsenates  h^T'drolyses  to  any  appreciable  extent. 


10. 

V. 

Bibliography . 

(1) .  J.  Ind . & iilng.  Chem.  13,  #11,  p.  1038  (Nov.  1921) 

(2) .  Koll.  Zeit.  28,  145-8  (1921). 

(3) .  Phil.  Mag.  U.,  425  (1906). 

(4) .  Zeit.  Anorg.  Chem.  67,107 . 


